This invention relates to a demodulator for signals subjected to simultaneous amplitude and phase modulation.
Digital or analog information is usually transmitted with modulation effected thereby on a carrier signal. It has been conventional to modulate either the amplitude or the phase alone of the carrier signal by information. For digital information, four-phase phase modulation is frequently resorted to. The quantity of information transmitted by four-phase phase modulation, however, is only two bits per symbol. When the transmission route is of a nature such that the route has little noise although the passband is severely restricted, four-phase phase modulation is not effective. The efficiency is improved by increasing the number of phases to 8, 16, and so forth. For digital amplitude modulation, the efficiency is likewise improved with an increase in the number of amplitude levels from four to 8, 16, and so on. On the other hand, it is already known that simultaneous modulation of amplitude and phase raises the efficiency as compared with the modulation of amplitude or phase alone with the same number of bits per symbol used, if the numbers are pertinently apportioned between amplitude levels and phases. The simultaneous modulation may either be amplitude phase keying (APK) or quadrature amplitude modulation (QAM). For analog information, the simultaneous modulation may be pulse amplitude and phase modulation (PAPM) as presented by G. R. Welti in "Communications Presentees au Colloque International sur les Telecommunications Numeriques par Satellite," Paris, 28-30 November 1972, under the title of "Pulse Amplitude-and-Phase Modulation". It is, however, necessary when demodulating the signals subjected to simultaneous amplitude and phase modulation to produce a reference signal on the receiving side, which serves as a phase reference synchronized with the original carrier signal used on the transmitting side and also as an amplitude reference proportional to the amplitude of the original carrier signal. Except for specific cases of amplitude phase keying, it is difficult to determine the phase reference from the received signals, as compared with determination of a similar phase reference from signals subjected solely to phase modulation. It is also difficult to determine the amplitude reference as compared with the case of sole amplitude modulation. The amplitude-and-phase modulation technique has accordingly been put into little practice, although it is an otherwise effective scheme.